Circadian rhythms of body temperature, locomotor activity, and drinking in many rats deteriorate or even disappear with age. The suprachiasmatic nucleus (SCN) in the anteroventral hypothalamus has been identified as the major circadian pacemaker in mammals. When fetal tissue containing the SCN is transplanted into young rats whose circadian rhythms have been abolished by SCN lesions, the rhythms gradually reappear. The anatomical and electrophysiological properties of the SCN undergo changes with age. Such changes may underlie the deterioration of circadian rhythms of overt behaviors in aged animals.In the present study, fetal tissue containing the SCN was implanted into the III ventricle of 21 aged rats. In seven, the grafts were either not viable or unobservable. In 50% of the remaining 14 hosts with viable grafts, circadian rhythms of body temperature, locomotor activity, and drinking were improved or restored. Transplantation of cerebellar grafts in 5 old rats with poor rhythms did not improve or restore the rhythms. This strongly suggests that the changes in behavior were specific to the grafted SCN tissue. The latency to improvement was from 10 days to 30 days postgraft. Improvements occurred in one, two, or all three rhythms in any particular aged host.Histological examination revealed that vasoactive intestinal polypeptide neurons were present in all successful grafts. The rare occurrence of VP and VIP efferent fiber outgrowth into the host tissue along with the fact that two successful grafts were located at the level of the median eminence, suggests that humoral factors released from the SCN grafts might have mediated the improvements of the rhythms in some of the successfully transplanted hosts.This work demonstrates for the first time that poor or absent circadian rhythms of body temperature, activity, and drinking in aged rats can be improved or restored following fetal SCN transplantation. Thus, the degeneration of circadian rhythms in aged rats is, at least in part, due to the deterioration of the aged SCN.